Sensing the Plant Surface Prior to Feeding and Oviposition: Differences in External Ultrastructure and Function Among Tarsi of Heliconius erato.

Adult foretarsi of Heliconius erato Linnaeus (Lepidoptera, Nymphalidae) are reduced in size and are not used for walking. Foretarsi of the female have specialized sensilla that are presumably used to identify the host plant, by drumming. The mid- and hind tarsi also bear sensilla in both sexes, but these have not been described in detail, nor has their chemosensory function been determined. We described and compared the tarsi of H. erato under light and scanning electron microscopy. Behavioral experiments showed that differences in the shape, number, and size of sensilla were related to feeding and oviposition behaviors. Two types of sensillum (chaeticum and trichodeum) were found in similar numbers and size on the mid- and hind tarsi of both sexes. Sensilla on the female foretarsi act in host-plant site selection, strongly affecting oviposition rates when isolated. Male foretarsi lack sensilla, which may have been selected against due to the absence of function and thus lost. Sensilla on the mid- and hind tarsi are involved in sugar detection in both sexes, responding to an effective dose of sucrose (ED50) near 0.01 M, and therefore might be used to identify food resources when the butterflies settle on flowers.